The present invention relates to a process for the treatment of waste water, in particular waste water from industrial processes containing at least hydrocarbons and salts.
The treatment or purification of waste water originating from industrial processes and containing at least hydrocarbons and salts (organic and/or inorganic) normally is a relatively expensive procedure. Environmental legislation nowadays puts stringent demands on the purification or waste water streams from industrial processes, particularly when the purified waste water is to be released into the environment. Accordingly, the choice of a purification method for industrial waste water is bound by practical, environmental and economic considerations.
One industrial process, wherein a relatively large amount of waste water is produced is the styrene monomer/propylene oxide (SM/PO) production process. In general such SM/PO process involves the steps of: (i) reacting ethylbenzene with oxygen or air to form ethylbenzene hydroperoxide, (ii) reacting the ethylbenzene hydroperoxide thus obtained with propene in the presence of an epoxidation catalyst to yield propylene oxide and 1-phenyl ethanol, and (iii) converting the 1-phenyl ethanol into styrene by dehydration using a suitable dehydration catalyst. In the last step water is produced. In addition to this reaction water organic by-products such as aliphatic and aromatic hydrocarbons, aldehydes, ketones, alcohols, phenols and organic acids are produced. The by-products are separated from the main products with the aid of clean water and the organic acids are neutralized using a basic aqueous solution, such as an aqueous sodium (bi)carbonate and/or sodium hydroxide solution. Furthermore, additional water is introduced with the air in the step (i) and as steam in step (iii) of the above process.
The waste water from an SM/PO production plant typically contains a total of from 1.0 to 3.5 wt % of non-salt organic compounds and from 3.0 to 6.0 wt % of organic salts. It may further contain up to 2.0 wt % of sodium carbonate and sodium bicarbonate and/or traces of sodium hydroxide, depending on the basic solution used in the neutralization of organic acids.
The input of clean water to an SM/PO plant can be up to tens of thousands kg per hour, while the output of waste water is normally about 50% higher than the imput of clean water. The waste water cannot be discharged without additional purification treatment. As has already been indicated above, however, the choice of a suitable purification treatment is limited due to all sorts of practical, environmental and economic considerations.
In GB-A-2,252,052 further information is given about prior art methods for treating waste water and about typical compositions of SM/PO waste water streams. The purification process disclosed in GB-A-2,252,052 involves freeze-concentration combined with salts-removal, whereby the waste water is separated into an at least two-fold concentrated waste product, salt crystals and a substantially pure water product.
The method disclosed in GB-A-2,252,052, however, still leaves room for improvement. Particularly the economics of a freeze concentration process are still not satisfactorily. Capital investment required for a freeze concentration process at present account for at least 10% of the total capital investment necessary for an SM/PO plant. It would thus be beneficial if a cheaper, but at least equally goodxe2x80x94in terms of purificationxe2x80x94alternative waste water purification treatment could be developed. Thus, it is an important object of the present invention to develop such alternative. A further object is to develop a waste water treatment which produces a clean water stream, which meets all requirements for discharge into the environment.
It has been found in accordance with the present invention that by applying a specific distillative treatment a very effective purification of the waste water is accomplished at reduced cost.
Accordingly, the present invention relates to a process for treating of waste water streams containing at least hydrocarbons and salts to yield a clean water product, a concentrated brine product and a hydrocarbon-rich product, which process comprises the steps of:
(a) feeding the waste water feed into a first distillation column at a stage in the range of from 0.05*n to 0.15*n from the top, wherein n represents the total number of theoretical stages of the first distillation column and has a value in the range of from 20 to 40;
(b) drawing off a vapour stream at a stage in the range of from 0.55*n to 0.75*n from the top and feeding this vapour stream into the bottom of a second distillation column having m theoretical stages, with m having a value in the range of from 3 to 10;
(c) drawing off the clean water product as the top fraction from the second distillation column and drawing off a bottom stream from the second distillation column, which is fed back into the first distillation column below the draw off of the vapour stream in step (b) at a stage in the range of from 0.60*n to 0.85*n from the top;
(d) drawing off the concentrated brine product as the bottom fraction of the first distillation column; and
(e) drawing off the hydrocarbon stream as the top fraction of the first distillation column.